C++ Micro Services: Example 7 - Spell Checker Client Module
Example 7 - Spell Checker Client Module

In this example we create a client for the spell checker service we implemented in Example 6. This client monitors the dynamic availability of the spell checker service using the Service Tracker and is very similar in structure to the dictionary client we implemented in Example 5. The functionality of the spell checker client reads passages from standard input and spell checks them using the spell checker service. Our module uses its module context to create a ServiceTracker object to monitor spell checker services. The source code for our module is as follows in a file called spellcheckclient/Activator.cpp:

#include "ISpellCheckService.h"
#include <usModuleActivator.h>
#include <usModuleContext.h>
#include <usServiceTracker.h>
// Replace that include with your own base class declaration
#include <iostream>
#include <cstring>
* This class implements a module that uses a spell checker
* service to check the spelling of a passage. This module
* is essentially identical to Example 5, in that it uses the
* Service Tracker to monitor the dynamic availability of the
* spell checker service. When loading this module, the thread
* calling the Load() method is used to read passages from
* standard input. You can stop spell checking passages by
* entering an empty line, but to start spell checking again
* you must un-load and then load the module again.
class US_ABI_LOCAL Activator : public ModuleActivator
: m_context(NULL)
, m_tracker(NULL)
* Implements ModuleActivator::Load(). Creates a service
* tracker object to monitor spell checker services. Enters
* a spell check loop where it reads passages from standard
* input and checks their spelling using the spell checker service.
* \note It is very bad practice to use the calling thread to perform a
* lengthy process like this; this is only done for the purpose of
* the tutorial.
* @param context the module context for this module.
void Load(ModuleContext *context)
m_context = context;
// Create a service tracker to monitor spell check services.
m_tracker = new ServiceTracker<ISpellCheckService*>(m_context);
std::cout << "Enter a blank line to exit." << std::endl;
// Loop endlessly until the user enters a blank line
while (std::cin)
// Ask the user to enter a passage.
std::cout << "Enter passage: ";
std::string passage;
std::getline(std::cin, passage);
// Get the selected spell check service, if available.
ISpellCheckService* checker = m_tracker->GetService();
// If the user entered a blank line, then
// exit the loop.
if (passage.empty())
// If there is no spell checker, then say so.
else if (checker == NULL)
std::cout << "No spell checker available." << std::endl;
// Otherwise check passage and print misspelled words.
std::vector<std::string> errors = checker->Check(passage);
if (errors.empty())
std::cout << "Passage is correct." << std::endl;
std::cout << "Incorrect word(s):" << std::endl;
for (std::size_t i = 0; i < errors.size(); ++i)
std::cout << " " << errors[i] << std::endl;
// This automatically closes the tracker
delete m_tracker;
* Implements ModuleActivator::Unload(). Does nothing since
* the C++ Micro Services library will automatically unget any used services.
* @param context the context for the module.
void Unload(ModuleContext* /*context*/)
// Module context
ModuleContext* m_context;
// The service tracker
US_EXPORT_MODULE_ACTIVATOR(spellcheckclient, Activator)

After running the CppMicroServicesExampleDriver program use the s command to make sure that only the modules from Example 2, Example 2b, and Example 6 are loaded; use the load (l) and un-load (u) commands as appropriate to load and un-load the various tutorial modules, respectively. Now we can load our spell checker client module by entering l spellcheckclient:

CppMicroServices-build> bin/CppMicroServicesExampleDriver
> l 
Starting to listen for service events.
> l spellcheckservice
Ex1: Service of type IDictionaryService/1.0 registered.
> s
Id | Name                 | Status
 - | dictionaryclient     | -
 - | dictionaryclient2    | -
 - | dictionaryclient3    | -
 - | frenchdictionary     | -
 - | spellcheckclient     | -
 1 | CppMicroServices     | LOADED
 2 | Event Listener       | LOADED
 3 | Dictionary Service   | LOADED
 4 | Spell Check Service  | LOADED

To trigger the registration of the spell checker service from our module, we load the frenchdictionary using the l frenchdictionary command. If the module from Example 1 is still active, then we should see it print out the details of the service event it receives when our new module registers its spell checker service:

CppMicroServices-build> bin/CppMicroServicesExampleDriver
> l spellcheckservice
> l frenchdictionary
> l spellcheckclient
Enter a blank line to exit.
Enter passage:

When we start the module, it will use the main thread to prompt us for passages; a passage is a collection or words separate by spaces, commas, periods, exclamation points, question marks, colons, or semi-colons. Enter a passage and press the enter key to spell check the passage or enter a blank line to stop spell checking passages. To restart the module, we must use the command s command to get the module identifier number for the module and first use the u command to un-load the module, then the l command to re-load it.

Since this client uses the Service Tracker to monitor the dynamic availability of the spell checker service, it is robust in the scenario where the spell checker service suddenly departs. Further, when a spell checker service arrives, it automatically gets the service if it needs it and continues to function. These capabilities are a little difficult to demonstrate since we are using a simple single-threaded approach, but in a multi-threaded or GUI-oriented application this robustness is very useful.

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